Tony Blair has said the EU is duty bound to block Theresa May’s negotiating goals as he made an impassioned call for new leadership to rescue the UK and Europe from economic disaster and the needless sacrifice of peace in Northern Ireland “on the altar of Brexit”.

The former prime minister dismissed the UK government’s strategy, which May is due to flesh out in a speech on Friday, saying it was doomed to fail despite interests on both sides of the channel in frictionless trade.

“Some in Britain believe that therefore Europe will bend in its negotiating stance and allow Britain largely unfettered access to Europe’s single market without the necessity of abiding by Europe’s rules”, Blair told an audience in Brussels. “This won’t happen because quite simply it can’t.”

To allow the UK such a deal would lead to the unravelling of the EU, Blair said at an event organised by the European Policy Centre thinktank. The onus now was on parliament to take the reins, defeat the government when it puts Brexit to the vote and open the way for a reconsideration that reflected the growing realisation among the electorate of the economic and political perils the country faced.

“We have months, perhaps weeks to think, plan and act”, he said. “The argument in Britain is far from over. It is in flux. See the speech of Jeremy Corbyn [endorsing a new customs union] this week.”

Blair said the problem of avoiding a hard border between Northern Ireland and the Irish Republic precisely illustrated the hopelessness of the government’s decision to leave the single market and customs union.

The questioning by some in recent weeks of the Good Friday agreement which has kept the peace in Northern Ireland since 1998 was sickening, he said.

He also said it was inevitable that those championing a hard Brexit would in time seek to complete “the Thatcherite revolution” to turn the UK into a low regulation, low tax pole apart from the European economic model.

In a section of the speech designed to turn heads in European capitals, Blair said: “Britain out of Europe will ultimately be a focal point of disunity, when the requirement for unity is so manifest. No matter how we try, it will create a competitive pole to that of Europe, economically and politically to the detriment of both of us.”

Blair therefore encouraged key figures in the EU to prepare a palatable way for the UK to come back into the fold should that reconsideration happen by proposing a comprehensive plan on immigration control “which preserves Europe’s values but is consistent with the concerns of its people and includes sensitivity to the challenges of the freedom of movement principle”.

“It doesn’t take a miracle”, Blair said. “It takes leadership. And now is when we need it … Those whose vision gave rise to the dream of a Europe unified in peace after centuries of war and whose determination translated that dream into practical endeavour, their ghosts should be our inspiration.

The EU and Britain are playing a high-stakes game of chicken | Anand Menon

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“They would not have yielded to fatalism and neither should we.”

Blair later told Sky News he believed there was a 50% chance of Brexit being eventually overturned.

The future of the Brexit negotiations was left in the balance on Wednesday when May dismissed the EU’s proposal for a “backstop” plan, under which Northern Ireland would in effect stay in the customs union and single market to avoid a hard border, as an attack on the UK’s constitutional integrity.

Talking to BBC Radio 4’s Today programme ahead of his speech, Blair said there was no solution without staying in the customs union and single market, and he offered support to John Major, his predecessor in Downing Street, who called for a Commons vote on a second referendum.

“Neither he nor I want to make her position difficult. This has gone far beyond that,” Blair said.

May is to make a speech on Friday in which she is expected to reaffirm her decision for the whole of the UK to leave the single market and customs union.

She will propose a system of “managed divergence” under which the UK could retain frictionless trade in some sectors by staying close to EU laws while changing regulations in other sectors to gain a competitive advantage.